The proposed research is subdivided into two areas, both of which utlize the goldfish Mauthner cell (M-cell) as a model. In the first aspect, basic mechanisms of synaptic interactions will be investigated. Specifically, the physiological chemical inhibitions of the M-cell will be studied, as will the role of he axon cap and its glial cell border in the generation of the field effects and the local regulation of membrane excitability. The input-output relations of the mixed electrotonic and chemical synapses between the M-cell lateral dendrite and VIIIth nerve afferents will be determined, and the effect of intracellular Ca ions injections on these mixed synapses will be studied, as will the hypothesis that the M-cell has Ca ions-dependent dendritic spikes which could modulate the properties of the synapses. The second aspect deals with the response of the M-cell to axotomy and/or partial deafferentation. Intracellular recordings will be used to analyze the alterations in M-cell membrane properties and synaptic interactions following transection of its axon or of different VIIIth nerve branches. The results will be correlated with light and electron microscopic morphological studies.